Production of para xylene



Dec. 12, 1950 R. G. DRESSLER PRODUCTION OF PARA XYLENE 4 Sheets-Sheet 2 Filed Aug. 29, 1947 cars TALL lm T/O/V TEMP.

% PARAFF/NS IN FEED 0 0 a a 0 0 7 6 5 4 o 2 ETHVL BENZE/VE IN FEED TEMP.

[NI/EN 70/? /Pussefl 6. Dress/er ATTORNEYS 9 1950 R. G. DRESSLER 2,533, 32

PRODUCTION OF PARA XYLENE Filed Aug. 29, 1947 4 Sheets-Sheet 4 a 9 k Y m \l =3 h Q: D g '35 Q u a: a k E Q :5. 1 3 I E 0R THO F I G 7 INVENTOR Passe 5 Dress/er ATTORNEYS Patented Dec. 12, 1950 UNITED STATES PATENT OFFICE PRODUCTION OF PARA XYLENE Russell G. Dressler, Louisiana, Mo., assignor to California Research Corporation, San Francisco, CaliL, a corporation of Delaware Application August 29, 1947, Serial No. 771,230

3 Claims. (Cl. 260-674) A complex hydrocarbon fraction with which the present invention is concerned primarily and from which para xylene may be recovered, typically contains only a minor proportion of para xylene. The proportion of para xylene in such mixtures seldom is more than 30% by volume and usually is less than about 21%by volume of the hydrocarbon fraction but should be more than about of the xylenes present in the mixture. The major portion of the mixture comprises aromatic hydrocarbons boiling within 11 F. of the para xylene and including from at least about 5% up to as much as 20% or more of ethyl benzene based on the entire hydrocarbon fraction. The ethyl benzene content may be from 50 to 100% by volume of the para xylene content. of these aromatic hydrocarbons at least about 50% by volume of the xylenes in the fraction is meta xylene,

with minor amount of ortho xylene not exceeding about 20% by volume. Additionally, the m7- lene fraction usually contains at least about 5% and up to 20% or more (based on the entire hydrocarbon fraction) Of unsulfonatable hydrocarbons of unknown constitution, generally identified as parafllnic, which may boil as much as 50 F. below the para. xylene and not more than about 20 F. above the para isomer. These parafflnic hydrocarbons may be present in amounts of from to 100% by volume based on the para xylene content and include acyclic saturated hydrocarbons which either boil within the range or form constant boiling mixtures with the xylene. Examples of such parafllnic hydrocarbons are various isomeric octanes and nonanes. The presence of cyclic paraflins, i. e., naphthenes boiling from 50 F. below to 20 F. above para xylene is not precluded.

An analysis of a xylene fraction typitying the above discussed compositions is:

Paramns and/or naphthenes 13 with which this invention deals are from 230 F. to about 300 F., more desirably boiling within the range of from 270 to about 300 F. and preferably within the range of from 2'70 to about 290 F.

The foregoing specific example has the following boiling range characteristics in an ASTM- D-86 distillation:

Initial $3 1 272 10 274 20 275 3'.) 275 40 275 50 276 (in 270 276 8(| 276 so 278 it"); 279 End Point... 314

The recovery of para xylene from such complex mixtures is not simple, since the presence of not only the isomeric xylenes but also parailins and aromatics isomeric to the xylenes, particularly of ethyl benzene, complicate and obscure the purification problem. Methods for recovering para xylene from its isomers have been proposed and prior proposals are of two general types, each of which has significant disadvantages and high cost factors. One type of proposal has involved extensive chemical alteration of one or more of the hydrocarbon components in the xylene system to afiord elimination and separation of the components. Such methods involve relatively expensive chemical conversions with attendant loss and normally require reconversion of the resulting chemical derivatives back to the desired hydrocarbon with additional loss at this stage as well as an overall useless consumption of chemical treating agent. Alternatively, physical methods heretofore proposed have recognized the complicating and obscure effects of aromatic and non-aromatic hydrocarbons in the xylene fraction and have attempted to solve this problem by removal thereof.

Spannagel Patent No. 1,940,065 allegedly recovers para xylene by crystallization but first purifies the xylene fraction by distilling off "any aliphatic hydrocarbons, ethyl benzene and the like, boiling below para and meta xylene, to avoid the complicating eilects of these impurities. In this patent ortho xylene also is removed and an intermediate meta, para xylene cut boiling Characteristic boiling ranges of xylene fractions II from 136-140 0., and evidently free of compli- 3 cating hydrocarbon impurities. is utilized in the crystallization step. Thus, in prior processes it appears that there has been an attempt to avoid unpredictable. obscuring and complicating effects of ethyl benzene by removal thereof as well as elimination of aliphatic hydrocarbon impurities. These purification treatments require extensive, elaborate and costly equipment particularly in the elimination of ethyl benzene by distillation.

Reference also has been made to the use of technically pure xylene of commerce for the separation of ortho, meta and para xylenes. As distinguished from crude xylenes the pure xylenes of commerce contain no more than 3% and usually less than 1% of paramns boiling within the range of from 279 to 285 F. Likewise, the ethyl benzene content of pure" xylenes of commerce sometimes called technically pure" is less than Contrary to the apparent beliefs of those skilled in the art it has been discovered that para xylene can be recovered in relatively good purity by crystallization from ortho and meta xylenes in the presence of from 5 to or more by volume of ethyl benzenes as well as in the additional presence of from 5 to 20% or more parafiins boiling within the range of from 230 to 300 F.

The unpredictability of this discovery can be better appreciated when it is noted that these hydrocarbons not only alter the crystallization temperature of para xylene by solvent action but that para xylene forms binary, ternary and quaternary crystals with various of the components and that the various components likewise form such complex crystals and eutectic mixtures with each other. The complexity and unpredictability of the system is illustrated by the following list of crystal types in the four component systemethyl benzene, ortho. meta, para xylene:

Para, ortho, xylene binary Para. meta, xylene binary Ortho, meta, xylene binary Ortho, meta, para xylene ternary Para xylene, ethyl benzene binary Para :wlene, ortho xylene, ethyl benzene ternary Para xylene, meta xylene, ethyl benzene ternary Ortho xylene, meta xylene, ethyl benzene ternary Ortho xylene, ethyl benzene binary Meta xylene, ethyl benzene binary Para xylene, ortho xylene, meta xylene, ethyl benzene quaternary where T: equals minimum temperature in R, x equals per cent parafllns in feed, Y equals per cent ethyl benzene in feed and Z equals per cent ortho xylene in feed.

4 (2) When the ortho xylene content is greater than one-half the percentage of meta xylene:

where T: equals minimum temperature in R, X equals per cent parafiins in feed, Y equals per cent ethyl benzene in feed and Z equals per cent of meta xylene in the feed.

(3) When the ortho xylene content equals onehalf the percentage of meta xylene:

where T2 equals minimum temperature in F., X equals per cent paraffins in feed and Y equals per cent ethyl benzene in feed.

In practicing the invention in its preferred embodiment, para xylene is produced and recovered from non-aromatic petroleum hydrocarbons. A suitable xylene fraction is obtained by aromatization, preferably by the so-called hydroforming process in which a naphthenic petroleum fraction is aromatized and xylenes are produced. This type of process is well-known in the petroleum industry. However, because the chemistry involved and the mixtures obtained are extremely complex, careful coordination of feed stocks and hydroforming conditions is necessary to obtain best results and to yield a preferred xylene fraction for recovery of para xylene in accordance with this invention.

The present invention is particularly adapted to the treatment of an equilibrium xylene mixture from hydroformed nonaromatic petroleum fractions. The term equilibrium xylene mixture is here utilized to designate a xylene fraction containing ortho, meta and para xylenes in the equilibrium proportions resulting from hydroforming or other suitable aromatization process, that is, in which the relative proportions are about 0 m p 2 6 2. The additional ethyl benzene and parafilns hereinbefore described are also present. Although, the invention is particularly adapted to the treatment of this specific type of mixture, it will be understood that the invention is also applicable to other xylene fractions of the compositions hereinbefore described. To avoid prolixity, the remainder of this description will be made with reference to xylene fractions derived from hydroforming operations.

In order to produce para xylene from non-aromatic petroleum hydrocarbons, proper selection of feed stocks and aromatizing conditions is important and essential to the most successful practice of the invention.

FEED STOCKS Naphthenic hydrocarbon mixtures from naphthene-type petroleum crude oils comprise one preferred type of feed stock. Such mixtures are normally termed straight run distillates" in the petroleum industry, although other aromatizable hydrocarbons or distillates may be substituted therefor. The hydrocarbons present in this preferred feed stock are believed to consist largely of cycle-aliphatic hydrocarbons with six carbon atoms in the cyclo-aliphatic ring and with aliphatic side chains attached to the ring. Some five and seven carbon atom cyclo-aliphatic rings may be present. Both the number of side chains and the length of each chain attached to the foregoing rings vary among the many compounds normally present in a petroleum hydrocarbon mixture. In general, these variables are a function of the average molecular weight or, more particuiarly, the boiling range and distillation curve of the petroleum fraction. A naphthenic hydrocarbon mixture consisting essentially of hydrocarbons having from six to twelve carbon atomsin the molecule and preferably composed at least predominantly of hydrocarbons containing from seven to eight carbon atoms at present is regarded as a more desirable feed stock. The fraction-selected desirably should boil fivithin the range of from about 180 F. to about 420i 1 k, and preferably from about 180 F. to about 320 F. In some instances an even more narrow cut boiling from 230 F. to about 275 Fjis preferred.

Open chain 'parafilnic hydrocarbon fractions of these boiling ranges are not precluded.

AROMATIZA'I'ION As previously set forth, an initial step in the exemplary process comprises aromatization of the particular petroleum feed stock selected. Where a naphthenic hydrocarbon mixture is utilized, the conversion of hydrocarbons to aro-' matics is believed to occur by dehydrogenation of the six carbon atom rings from cyclo-aliphatic to aromatic while leaving alkyl groups attached to the residual nucleus. For example:

HzC (DH-CH3 CH3 3H: HiC CH-CH: CH3

Ortho dimethyl cyclohexane Zinc C H- 0 H:

Meta dimethyl cyclohexane Pam dimcthyl cyclohoxane CHrC H1;

2731 CHzCHa H2O CH2 3H: HzC CH2 Ethyl cyclohexane Isomerization of any C7 alicyclic rings present and dehydrogenation to aromatic compounds also is believed to occur. Likewise, C5 alicyclic rings containing side chains are converted to aromatics by isomerization and dehydrogenation. These various reactions represent an over-simplification oi the aromatization reactions which may actually occur, since de-alkylation and shortening of side chains as by cracking undoubtedly take place. In any event, the aromatization reaction product comprises a highly complex mixture of aromatics and also contains non-aromatic hydrocarbons, such as saturated or unsaturated parailins and naphthenes. The overall com matic fractions from open chain paramnic hy-.

8 plexity of the mixture and the relative proportion of the above-mentioned non-aromatic components depend upon the effectiveness of the particular aromatization processes well as upon the specific hydrocarbon feed stock selected. It 18;; for this reason that a highly naphthenic hydro-" carbon feed stock boiling within the ranges prepreferred, since the reacviously disclosed are tion products therefrom are better adapted to subsequent processing steps involved in the production of isomeric xylenes. However, sible, but less desirable, to obtain operative arodrqcarbons by known reactions, such as dehydrogen'ationand cyclization illustrated by the fol lowing reactions:

Processes for eifecting such aromatization' re-, actions and catalysts therefor are known in the petroleum art.

' dehydrogenation reactions similarto the i'oregoing. These various known processes may be utilized within the broader aspectsgof this in= vention and are embraced within the, term aromatization" as used in the present specification.

The preferred aromatization process known as fhydroforming" is characterized by aromatization in the presence of controlled amounts of hydrogen and a vanadium oxide or molybdenum oxide catalyst. As an example of the preferred process, a hydrocarbon feed, such as a naphthenic petroleum distillate, boiling within the range of 180 F. to 320 F., and obtained, for instance, by fractional distillation of a crude petroleum (from Kettleman Hills Oil Field in California) is passed at from about 900 F. to about 1200 R, desirably about 1000" F., over a vanadium oxide-alumina or molybdenum oxide-alumina catalyst. Space rate desirably is from 0.1 to about 2.0 volumes of liquid hydrocarbon feed per volume of catalyst per hour, and it is preferred to maintain a partial pressure of hydrogen in the reaction zone of from about 30 to about 300 pounds per square inch. s The reaction product from such a hydroforming operation will contain not only the desired xylenes and additional aromatic hydrocarbon but also aliphatic hydrocarbons boiling over a wide range, including C4 and like materials. Initial! therefore, it is necessary to recover a xylene fraction from this reaction mixture.

In the drawing:

Fig. 1 is a schematic flow sheet of a typical process and suitable apparatus for practicing the process of this invention.

it is pose 1 Likewise, aliphatic oleflns are convertible to aromatics by known cyclization and 7 Figs. 2 and 3 illustrate graphically the effect of parafllns upon crystallization temperature and recovery of para xylene.

Figs. 4 and 5 illustrate the effect of ethyl benzene on crystallization temperature and recovery of para xylene; and

Fig. 6 reveals the influence of relative proportions of the xylene isomers on crystallization recovery of para xylene.

Fig. '7 shows the effect of ortho xylene concentration on optimum crystallization temperatures and at different ratios of ortho to meta xylenes.

Referring to Fig. 1 of the drawing, a naphthenic hydrocarbon feed is introduced by way of line In to a hydroforming unit If and non-aromatic petroleum hydrocarbons such as the naphthenic petroleum distillate boiling within the range of 180-320 R, as previously described, is converted to a complex aromatic hydrocarbon fraction. Desirably the particular hydroforming operation is that previously described and exemplified as a preferred process. The hydrocarbon effluent flows by way of line l2 to a fractionating column l3 where separation is effected. As here shown, the fractionation is effected in a single column although a multiplicity of fractionating units may be utilized. Cr and lighter hydrocarbons are taken as overhead through line l4 while C5, C6 and C7 hydrocarbon fractions are removed separately as side streams by way of lines l5, l6 and I! respectively. C9 and heavier hydrocarbons are discharged as bottoms by way of line 18. The xylene rich hydrocarbon mixture from which para xylene is to be recovered is withdrawn from fractionating column l3 by way of line I9 and flows through cooler 2| to surge tank 22.

The Xylene rich hydrocarbon fraction containing paraflins and ethyl benzene, as herein before described, flows from surge tank 22 to and through the para xylene recovery system. Although not essential to operativeness of the process, it will be found highly desirable in various instances to adjust the ratio of ortho to meta xylene in this xylene feed stock in order to enhance recovery of the para isomer. The ortho xylene content of a fraction prepared by hydroforming is less than the preferred ratio and as here shown ortho xylene is added to the hydrocarbon mixture in surge tank 22 by line 23, and

the ortho to meta xylene ratio is thereby adjusted to approximately 1:2. The blended hydrocarbon mixture so formed then flows by way of lines 24 and 26 through heat exchanger 21 where the temperature of the mixture is initially lowered most desirably by indirect heat exchange with mother liquor from the crystallization operation. This mother liquor flows through inlet and outlet conduits 28 and 29, but for purposes of simplicity connections with the mother liquor lines are not shown.

It has been found that recovery of para xylene can be enhanced and superior results obtained by avoiding shock cooling of the entire xylene stream or conversely by maintaining the stock at crysta lizing temperature for a substantial length of time to allow growth of crystals and insure dissipation of any adverse effects of localized sho"k cooling. Best results have been obtained with at least ten minutes and more desirably with twenty minutes or more residence time at crystallizing temperatures. As here shown the xylene stream flows into a suitably heat insulated soaking or crystal growing tank 3i where it is reduced to crystallizing temperature by mixing with previously' cooled xylene stock. The xylene stock is retained at crystallizing temperature until the desired crystal form is obtained, that is, until shock crystals are largely removed by remelting and recrystallization or by equilibrium exchange with larger crystals which will be retained and recovered satisfactorily in subsequent filtering operations. Generally, a residence time of about twenty minutes is preferred.

Extremely rapid cooling of the incoming xylene stream adversely effects para xylene recovery, tends to lower the purity of product, and

' produces undesirably fine crystals which can be separated from the mother liquor only with great difliculty if at all. Thus. a. cooling rate in the order of 50 F. a minute in a batch process produces such adverse effects, where as a cooling rate through the crystallization temperature range in the order of l to 10 F. a minute gives a good ield of filterable crystals of relatively high purity. More desirably, a cooling rate below about 5 F. a minute through the crystallization temperature range may be utilized.

Crystallizing temperature is maintained in soaking tank 3| by circulation of a xylene side stream through chillers by way of line 32. Thus, circulation pump 33 forces the xylene through temperature-controlled chillers 34, 3S and 31 connected in parallel, as shown, by valve-controlled inlet lines 38, 35 and 4!. Desirably, circulation pump 33 is designed and controlled to force the xylene mixture through the chiller tubes at a sufficient velocity and under adequate pressure to cause turbulent flow. The term "turbulent flow here is used in the commonly accepted hydraulic sense. Such turbulent flow is adapted to prevent or minimize localized shock cooling of the xylenes at the surface of the heat exchange tubes in coolers 34, 36 and 31. Additionally, crystal growth and adherence on the walls of such heat exchange tubes is reduced to a minimum by the use of high velocities, especiall those exceeding the minimum for turbulent flow. For example, supercooling may be effected in the heat exchange tubes and the supercooled liquid returned to the crystallization tank before crystal formation is completed. After reduction to a crystallization temperature at least as low as that to be maintained in soaking tank 3|. the xylene mixture is passed through chiller discharge lines 42, 43, 44 and return header 46 to the crystal soaking or growing tank. The chilled xylene mixture is dispersed with the crystal slurry in tank 31 and an equilibrium temperature condition is reached therewith.

Any suitable refrigerant is supplied to the chillers by way of inlet header 25 and outlet 30. Liquefied ethylene, ethane or methane are examples of suitable refrigerants. As here indicated, temperature controls 35 are provided in the refrigerant discharge line of each of the chillers to regulate the flow of refrigerant therethrough. Desirably, these controls are responsive to the temperature of the xylene mixture in discharge lines 42. 43 and 44 respectively.

Upon completion of the crystal growing operation in tank 3|, the slurry of para xylene crystals in the remaining liquid hydrocarbon mixture isconveyed by suitable means, as indicated by line 41, to a crystal separation and recovery unit. As illustrated herein, crystal separation and purification are effected by a combination of centrifugal filters and an agitated tank washer. Initially the cry tals in slurry from aasaesa tank 3| are separated in a centrifugal filter 48 at a temperature of from about 75-F. to about -l20 F., more desirably -80 F. to l R, and preferabl from -80 F. to T2 as previously defined, and conveyed as indicated by line 49 to crystal washer 5| .5 Any suitable washing fluid may be utilized, such as isopentane, alcohol or the like, but as here shown a para xylene saturated hydrocarbon mixture is introduced by way of valve-controlled line 52 with the slurry and the mixture intimately contacted by agitators 53. The resultant slurry flows through outlet line 54 to a second centrifugal filter 56. In order to maintain and control the temperature in washer 5!, a portion of the washing liquid in stream 54 is by-passed through valvecontrolled line 51, heater 58 and return line 59 to washing tank 5!]. Steam is supplied to heater 58 as indicated by inlet and outlet lines 6! and 62.

The crystal slurry from washer at is separated in the second stage centrifugal filter 55, and the purified crystals removed and transferred to melting tank 63 as indicated by line 6 3. The filtrate from this second stage separation is discharged b way of line 68. This filtrate comprises a xylene fraction saturated with respect to para xylene at filtration temperature. A portion thereof flows by way of valve-controlled line 52 to be utilized as the washing liquid in tank 5i. The remainder of the filtrate from unit 56 passes by way of recycle line bl through heat exchanger 21, and preferably is blended with the xylene feed stock, before it is introduced into soaking tank 3|.

In some instances, it will be found desirable to minimize crystal formation in chillers 3'3, 36 and 3? b recirculating the filtrate of recycle line til through the heat exchanger tubes 38, 39 and 3! together with, or in lieu of, xylenes from crystal forming tank 3i. A by-pass line tla from recycle line 67 to pump 33 is provided for this purpose.

Purified crystals of para xylene in tank 53 are melted and passed to storage 68 by way of line 6.39. A portion of the melted stock is by-passed through valve-controlled line ll, heater l2 and return line 73, the heated xylene serving to melt crystals fed to the system. Heat is supplied by hot water or any other suitable fluid introduced through line 74 and discharged through line l5.

The two-stage filtration and crystallization system will be operated with first-stage filter 38 maintained at a lower temperature than second stage filter 56. A portion of the crystals discharged from washer bl is allowed to melt so that the filtrate from unit 56 is para xylene of the desired purity, thereby furnishing a wash liquid rich in para, xylene by way of valve-controlled line 52 for removing entrained less pure mother liquor from the crystals in washer 5i. Temperature in such a washing operation may be from about F. to +35 F., although lower temperatures may be used depending upon purity and yields desired.

Mother liquor from first stage filter 138 is discharged by way of outlet conduit ll, and in the embodiment here illustrated passes to fractionating column 18 wherein an ortho xylene fraction is separated by distillation.

In this distillation relatively high purity ortho xylene fraction (for example, 95% or higher) can be obtained by superfractionation, which is a preferred type of operation for the present invention. The ortho xylene is removed from the distillation as a bottoms fraction by way of discharge line 19. A portion of the ortho xylene desirably is recycled by way of valve-controlled line 23 to feed surge tank 22 in an amount sufllcient to adjust the ortho xylene content of the feed as previously disclosed herein. The remainder of the ortho xylene flows to storage by way of valve-controlled line 8|. Overhead from superfractionator 16 passes by way of line 82 to storage 83. This overhead fraction consists of a mixture of xylenes, primarily meta xylene with minor amounts of ortho and para xylenes as well as with paraflins and ethyl benzene contained in the original feed stock.

With respect to the separation of an ortho xylene fraction by distillation and superfractionation, it should be noted that it will be necessary to maintain the non-aromatic hydrocarbon content of the xylene fraction supplied to superfractionator 78 below about 15% by weight. When necessary this initial purification ma be effected in any suitable manner as, for example, by an initial extractive distillation of the xylene, or by liquid phase selective solvent extraction or the like. The superfractionation itself requires a highly efllcient fractionating column. One equivalent to 35 theoretical plates is necessary for practical operation, more desirably about 45 and preferably about 60 theoretical plates are utilized. Reflux ratios on distillate of from about 7:1 to about 12:1 have been found satisfactory. Very close temperature regulation is important, but the distillation is so sensitive that control by temperature responsive device has been found to give inefficient though operable separation. A preferred method of superfractionation is to operate the fractionating unit continuously at a given constant feed rate while (1) removing overhead distillate and bottoms at a constant ratio corresponding to the feed rate and in a relative proportion such that the desired purity of the ortho xylene may be maintained, and (2) maintaining a constant volume of liquid and still bottoms by controlling the rate 'of heat input thereto. Maintenance of the constant volume of bottoms may be efifected, for example, by a constant level control which increases the amount of steam admitted to the still heating unit when the level of the still bottoms begins to rise, and decreases steam input when the volume of bottoms begins to drop below the predetermined level. With the benefit of the foregoing instructions, those skilled in the art can effect superfractionation of a mother liquid boiling within the range of, for example, 275-295 F. and having a non-aromatic hydrocarbon content of less than about 15% by Weight.

To further illustrate the invention and guide those skilled in the art in the practice thereof. data showing efiective recovery of para xylene in the presence of different amounts of parafilns and at different temperatures are presented graphically in Fig. 2. The feeds B and C referred to in Fig. 3 had the following composition:

paramns tend to decrease para xylene recovery at any given temperature, but that this decrease in recovery is avoidable by further reducing crystallization temperature within the limits and in the manner herein disclosed: that is, by lowering the temperature 0.3 F. for each per cent of Darafilns present.

Figs. 4 and 5 establish the effects of the presence of ethyl benzene and show that it tends to decrease p-xylene recovery at any given temperature to a greater extent than do the paraillns. Likewise, the data illustrate that this decrease in recovery is avoidable by reducing crystallization temperature within the limits and in the manner herein disclosed, that is, by lowering crystallization temperature as previously disclosed about 1.4 F. for each percentage of ethyl benzene present in the feed.

Figs. 6 and 7 show the effects of ortho xylene to meta xylene ratio on para xylene recovery and on optimum crystallization temperature. The data of these two figures are based on compositions containing para xylene in excess of the xylene eutectics. Thus, when the ortho-meta xylene ratio is less than one-half, crystallization temperature should be decreased about 0.65 F. for each per cent of ortho xylene present. When the ratio of ortho to meta xylene is greater than one-half, the optimum crystallization temperature for any given feed containing para xylene in excess of the eutectic proportion should be increased about 1.3 F. for each per cent of ortho xylene present in excess of 33, based on the xylenes. Again, when the ortho to meta xylene ratio is the optimum 1:2, then the most desirable crystallization temperature is -84.5 F. decreased by the correction factors previously disclosed for parafiin and ethyl benzene contents only.

An exemplary process was carried out and data obtained in a simplified aparatus consisting of a fritted glass filter surrounded by a cooling bath to maintain the filtration at specified temperatures. Crystals were filtered from the mother liquor by applying vacuum, and the crystal cake of para xylene was air-dried for a measured time interval. Crystals were weighed and purity determined by the freezing point method. To regulate the sweating of the crystal cake and eliminate ice formation on the filter, the air used for drying was first chilled with an alcohol solid CO2 bath. In these runs the percentage of parafllns was varied by adding the unsulfonatable residue (that is, the parafiinic hydrocarbons) of a xylene fraction formed by hydroforming a petroleum hydrocarbon fraction as previously described herein. By utilizing this particular mixture of paraflins, representative results were obtained without the necessity of identifying the exact composition and proportions of the different paraffinic components. Tables I and II give the results of representative runs made in the foregoing manner, Table I showing the effect of parafllns:

TABLE I Efl'ect of paraflins on the recovery 0] para xylene Feed B Feed F 12 Conditions:

(1) Charge; 50 g. ieed (2) Vacuum; fiowmeter with #30 orifice,

20 cm. Hg (3) Drying air cooled in alcohol CO: bath (4) Cake air dried 2 minutes successively increased percentages of ethyl benzene obtained by addition of ethyl benzene to the original charge stock.

TABLE II Efiect of ethyl benzene on recovery of para xylene Charge stock:

Per cent ethyl benzene 14.0 Per cent o-xylene 6.1 Per cent m-xylene 49.3 Per cent p-xylene 19.1 Per cent parafiins 11.5 Conditions:

( 1) Charge; 50 cc. toluene plant topping still overhead (2) Vacuum; fiowmeter with #30 orifice,

20 cm. Hg (3) Drying air cooled in alcohol CO: bath (4) Cake air dried 2 minutes Crystalliz- Time at p-Xylcnc iuthyl T Cooling c h i -Xy1enc Benzene 2 5m Time T ifp. Cry s tals vexed Percent F. Percent Percent 14. 0 -15 35 10 1s. 0 4a. a 28.4 30 30 14.3 33.3 38.6 -75 2a 30 m5 27.6 14.0 20 20 711 58.5 14.0 -90 2o 30 76.8 58.5 21.3 -90 1s 5 76.0 50.2 38.6 -90 22 5 10.4 31.4 38.6 -90 20 10 72.0 36.8 14. 0 -1o2 35 5 14. 0 as. 7 28.4 25 10 71.0 50.0 38.6 -105 25 5 69.2 41. 0

A second series of exemplary runs was made with centrifugal separation of para xylene crystals. The filtration was effected in equipment consisting of a perforated basket centrifuge lined with muslin. An agitated chilling vessel was provided for cooling the feed stock by internal refrigeration by direct addition of Dry Ice. In addition to the mixing eifected by evaporated CO2, mechanical agitation was utilized to aid in controlling the temperature of the charge stock and in reducing agglomeration of the solid C02.

The para xylene crystal slurry was fed by gravity into the centrifugal filter, and a pump was provided for recirculation of the cooled mother liquor from the filter back to the agitating vessel. The centrifugal pump, agitator and pipe lines were suitably insulated to maintain low temperatures. Means for measuring temperatures in the agitator and of the inlet and outlet of the centrifuge were provided. In operation, the whole system was gradually cooled to the desired crystallization temperature by addition of solid CO:

to the agitator and continuous recirculation of the xylene mother liquor through the agitator and centrifugal filter. Purification or the crystals in situ was eitected in two stages; first, extraction of impurities by circulation of the mother liquor through the filter cake for a substantial period after crystallization temperature is reached; and secondly, by drawing on the mother liquor and allowing the filter cake to rise in temperature sufiiciently to "sweat out hydrocarbon impurities while continuing operation of the centrifuge to remove liquefied impurities so released. Data irom these runs are given in Table III:

TABLE HI Charge stock:

Per cent ethyl benzene 14.0 Per cent o-xylene 6.1 Per cent m-xylene 49.3 Per cent p-xylene 19.1 Per cent parailins 11.5

Final Cryatalliz- R. P. M. Purity Recovery ing. Temp. gg: of Centriof of n-Xylene T. Permd i'uge p-Xylene Charged F. Minutes .Per cent Per cent 70 93 9. 7 80 10 87. 2 30. 7 75 10 85. 5 48.0 90 83. 5 58.4 --84 15 88. 2 56. 8 87 86. 3 58. 7 91 12 87. 0 l 30. 5 -95 82 .92. 0 l 42. 2 -90 70 95. 7 57. 5 -90 58 88 58 90 93 96 1 50. 6 90 69 98 58 -80 50 90 F) --l00 4 68 95 59 1 Equipment was cork insulated for this and ensuing runs. Low recovery due to increased speed of centrifuge.

I Some mechanical loss of product from centrifuge.

3 Centrifuge modified to germit measurement of R. P. M. F or this 1liunPtllrz speed of the na] centrifuge period was increased to I This period was reduced by increasing the air flow through the centrifuge.

It is readily apparent from the foregoing description that various modifications of the process can be made within the spirit of the present invention and the scope of the appended claims. For the sake of simplicity and clarity, apparatus has not been shown in detail in the drawing but is illustrated only as to major unit operations in the process. Many detailed pumps, valves, condensers, heat exchangers, temperature controls and the like have been omitted. since any suitable Iorm of apparatus incorporating these features can be supplied in obvious manner by those skilled in the art.

' I claim:

I. A process of recovering para xylene from when the ortho xylene content is less than onehalf the percentage of meta xylene, and

- 14 when the ortho xylene content is greater than one-half the percentage of meta xylene and when the ortho xylene content is approximately one-half the percentage of meta xylene, where X and Y are the percentages 01 paraillns and ethyl benzenes respectively based on the feed, and Z in Formulas 1 and. 2 is the percentage of ortho xylene and meta xylene, respectively, based on the isomeric xylene contained in the feed.

2. A process of recovering para xylenes from a xylene fraction containing more than 10% and less than about 130% by volume of a para xylene, said xylene fraction comprising ortho and meta xylenes and containing from approximately 50% to approximately 100% by volume based on the para xylene of ethyl benzene and from at least about 25% up to about 100% by volume based on para xylene of paralllnic hydrocarbon impurities boiling within the range of from about 50 F'. below to about 20 F. above the boiling point of para xylene, said process comprising selectively separating para xylene crystals by chilling said xylene fraction to crystallizing temperature and recovering para xylene crystals from mother liquor at a temperature of from about F. to about T2 where T2 is determined by the follow ing equations:

where the ortho xylene content is less than onehalf the perecentage of meta xylene, and

when the ortho xylene content is greater than one-half the percentage of meta xylene, and

when the ortho xylene content is approximately one-half the percentage of meta xylene, where X and Y are the percentages of parafilns and ethyl benzene respectively based on the feed, and Z in Formulas l and 2 is the percentage of ortho xylene and meta xylene, respectively, based on the isomeric xylene contained in the feed.

3. A process of recovering para xylenes from a xylene fraction containing more than 10% and no more than about 21 by Volume of para xylene,said xylene fraction comprising ortho and meta xylenes and containing from approximately 50 to approximately by voume based on the para xylene of ethyl benzene and from at least about 25% up to about 100% by volume based on para xylene oi parafilnic-hydrocarbon impurities boiling within the range of from about 50 F. below to about 20 F. above the boiling point of para xylene, said process comprising selectively separating para xylene crystals by chilling said xylene fraction to crystallizing temperature and recovering para xylene crystals from mother liquor at a temperature of from about -80 F. to about T: where T2 is determined by the following equations:

when the ortho xylene content is less than onehalf the percentage of meta xylene, and

when the ortho xylene content is greater than one-half the percentage of meta xylene, and

Tz=-84.5 F.-0.3X-i.4Y when the ortho xylene content is approximately spasm-n 15 16 one-half the percentage of meta/xylene, where UNITED STATES PATENTS X and Y are the percentages of paraflms and N b N Date ethyl benzene respectively based on the feed, and 5 gg z et aL Dec 19' 1933 Z in Formulas 1 and 2 is the percentage of ortho 2398526 Greenburg Apr. 16 1946 xylene and meta xylene, respectively, based on 5 2404902 claussen July 1946 the Ewen) Xylene ntained in the feed- 21435792 McArdle et; a1. II: Feb. 10' 1948 RUSSELL G. DRESSLER.

' OTHER REFERENCES REFERENCES CITED Kravchenko: Chem. Ab., vol. 36. 4016 (1942). The following references are of record in the file of this patent: 

1. A PROCESS OF RECOVERING PARA XYLENE FROM ORTHO AND META XYLENES IN THE PRESENCE OF FROM AT LEAST 5% BY VOLUME (BASED ON THE ENTIRE HYDROCARBON FRACTION) OF ETHYL BENZENE AND IN THE PRESENCE OF AT LEAST 5% BY VOLUME (BASED ON THE ENTIRE HYDROCARBON FRACTION) OF PARAFFINS BOILING WITHIN THE RANGE OF FROM 230 TO 300*F., WHICH COMPRISES SEPARATING A CRYSTALLIZED PARA XYLENE FRACTION AS A SOLID PHASE FROM LIQUID HYDROCARBONS OF SAID MIXTURE AT A TEMPERATURE BELOW -75*F. BUT ABOVE T2 WHERE 